I can't tell from your post if you are wilderness trekking (what this site is all about) or doing budget travel with occasional camping. If the latter, you might get better information from budget travel sites like Lonely Planet.

Just to add to the above advice, all excellent, except the 90L pack: I go out backpacking (wilderness trekking) for 7-9 days at a time with a 30L pack, total starting weight (including 9 days of food) of 24 lbs. (11 kg). I also have traveled in Europe for 3 months at a time with only 2 changes of clothing, which was plenty. Quick-dry fabrics are the answer. Packing light is more fun!

The three most important aspects of a backpack are fit, fit and fit. The pack must fit your gear, fit your body and be comfortable for you to carry with your gear inside. Backpack fit is as important as shoe fit!